Solar energy can be used and stored by the efficient production of long-lived photo-induced charge separation--a state achieved in photosynthetic systems by the formation of a long-lived radical pair. A number of artificial systems have been reported that efficiently undergo photochemical charge transfer, unfortunately, the thermal back electron transfer often proceeds at an appreciable rate, limiting the utility of these systems. What is needed is a systems which has very efficient photoinduced charge transfer, and forms a charge-separated state which is long lived in air. The charge separation in these systems typically involves a redox reaction between a photoexcited donor and a suitable acceptor, resulting in the production of radical ion pairs illustrated by the formula: EQU D+hv.fwdarw.D* (1a) EQU D*+A.fwdarw.A.sup.- +D.sup.+ ( 1b) EQU D.sup.+ +A.sup.- .fwdarw.D+A (2)
The cation and anion generated in this way are better oxidants and reductants, respectively, than either of the neutral ground-state molecules. To harvest the light put into this system, the oxidizing and reducing power of the photogenerated species must be used before the electrons are transferred back (equation 2) generating the starting materials. It is desirable to control this photochemically unproductive thermal fast back electron transfer reaction. One method has been to incorporate the donors and acceptors into solid matrices.
Compounds which can carry out reduction reactions, using hydrogen gas as their reducing equivalents, are useful as catalysts for the conversion of mixtures of hydrogen and oxygen to hydrogen peroxide. Hydrogen peroxide is a very large volume chemical. The United States annual production is greater than 500 million lbs. Several processes have been patented for the production of hydrogen peroxide, which depend on the two following reactions. The goal is to promote reaction (1) and retard reaction (2). EQU H.sub.2 +O.sub.2 .fwdarw.H.sub.2 O.sub.2 ( 1) EQU H.sub.2 O.sub.2 +H.sub.2 .fwdarw.2 H.sub.2 O (2)
A number of catalysts for this conversion have been reported including both homogeneous and heterogeneous catalysts. The higher yielding homogenous catalysts, for example those disclosed in U.S. Pat. Nos. 4,800,075; 4,046,868; 4,668,499; 5,254,326; 5,194,067; 5,041,680; 5,039,508; 4,994,625; and 4,897,252, are limited by the difficult step of separating the hydrogen peroxide from the reaction mixture. The heterogenous catalysts allow easy isolation of the hydrogen peroxide but require high pressure and exhibit short catalytic lifetimes, such as the one disclosed in U.S. Pat. No. 4,832,938 ("the DuPont patent"). The DuPont catalyst system consists of colloidal metal particles bound to inert supports, such as silica, alumina and carbon. These materials are prepared by first generating an aqueous suspension of the desired metal colloid (with a set ratio of Pt to Pd) and then spray drying this solution onto the inert support. The resulting solid is heated in hydrogen to 200.degree. C. to form the catalyst. Bromide or chloride promoters as well as phosphonic acids were added to the system. The role of the promoters and phosphonic acid are not well defined. All of the chemistry occurs at the colloid particle. The peroxide production reaction involves treating an aqueous suspension of the catalyst with high pressures of hydrogen and oxygen (1000-2000 psi). The hydrogen adds to the surface as does the oxygen. The problem with this system is that the same particles that are good at forming peroxide (equation 1) are good at converting peroxide to water (equation 2). A wide range of different ratios of Pt:Pd was investigated to develop the most active catalyst. There is no reported analytical data on the materials of the DuPont patent. The ratios are calculated based on what goes in the flask and not what comes out. They do not report any evidence as to whether the Pd and Pt are uniformly mixed in their colloidal particles or they form separate species on the surface of the support. What is needed is a heterogeneous catalyst with an improved yield.
The compositions of the present invention are capable of producing a sustained photoinduced charge separation state which renders the compositions useful in solar energy conversion and storage. In addition, the compositions permit reduction of various metal ions to produce the zero-valence metal in colloidal form entrapped in the matrices of the compositions. These latter matrices containing the zero-valence metal have a variety of uses such as in the decomposition of water to yield hydrogen gas and the sensing of oxygen. In addition, the zero-valence metal matrices can be used in catalysis, as for example in the production of hydrogen peroxide and the oligomerization of methane to form higher hydrocarbons.